Reflective power monitoring for microwave applications

ABSTRACT

A system and method for supplying microwave energy to tissue for microwave therapy includes an electrosurgical generator having an output for coupling to a surgical instrument. The electrosurgical generator includes a microwave energy source and a controller for controlling the operation of the electrosurgical generator. The surgical instrument, coupled to the electrosurgical generator, includes a microwave antenna for delivering microwave energy from the microwave energy source. The controller of the electrosurgical generator is operable for causing the electrosurgical generator to apply at least two pulses of microwave energy.

BACKGROUND

1. Technical Field

The present disclosure relates generally to medical/surgical ablation procedures. More particularly, the present disclosure relates to devices and microwave radiation delivery procedures utilizing microwave antenna assemblies and methods of controlling the delivery of microwave radiation to tissue.

2. Background of Related Art

In the treatment of diseases such as cancer, certain types of cancer cells have been found to denature at elevated temperatures (which are slightly lower than temperatures normally injurious to healthy cells). These types of treatments, known generally as hyperthermia therapy, typically utilize electromagnetic radiation to heat diseased cells to temperatures above 41° C. while maintaining adjacent healthy cells at lower temperatures where irreversible cell destruction will not occur. Other procedures utilizing electromagnetic radiation to heat tissue also include ablation and coagulation of the tissue. Such microwave ablation procedures, e.g., such as those performed for menorrhagia, are typically done to ablate and coagulate the targeted tissue to denature or kill it. Additionally, microwave therapy may be used in the treatment of tissue and organs such as the prostate, heart, and liver.

One advantage of microwave therapy is that microwave energy is able to non-invasively penetrate the skin to reach underlying tissue. Unlike low frequency RF therapy, which heats tissue with current, microwave therapy heats tissue within the electromagnetic field generated by a microwave antenna. The electromagnetic field generated by the microwave antenna generates a predictably large and/or uniform ablation region.

A second advantage of microwave therapy is that energy is rapidly delivered to the target tissue resulting in the reduction of surgical procedure time. During a typical surgical procedure microwave energy rapidly heats tissue to a target temperature and maintains the tissue above the target temperature for a required period of time.

Rapid delivery of heat to tissue may also result in the unwanted heating of healthy tissue or overheating of the target tissue. Unwanted heating of healthy tissue may be the result of creating an electromagnetic field larger than is required or excessive energy delivery. Overheating of tissue may result from excessive energy delivery or inconsistent heating of the target tissue.

Thus, the non-invasive use of microwave energy requires a great deal of control. This is partly why a more direct and precise method of delivering microwave radiation has been sought. The present disclosure provides a system for supplying microwave and various methods of delivering microwave radiation to tissue.

SUMMARY

The present disclosure relates generally medical/surgical ablation procedures. More particularly, the present disclosure relates to monitoring characteristics of microwave radiation delivery procedures utilizing microwave antenna assemblies configured for direct insertion into tissue and methods of controlling the delivery of microwave radiation to tissue.

A system for supplying microwave energy for microwave therapy of the present disclosure comprises an electrosurgical generator including a microwave energy source and a controller for controlling the operation of an electrosurgical generator. The electrosurgical generator includes an output for coupling to a surgical instrument which includes a microwave antenna for delivering microwave energy. The controller being operable for causing the electrosurgical generator to apply two or more pulses of microwave energy to the tissue.

The controller may be configured to measure an electrical characteristic of at least one of the at least two pulses of microwave energy. Controller may be responsive to the measured electrical characteristic of at least one of the at least two pulses of microwave energy for determining at least one pulse parameter of a subsequent microwave energy pulse. The pulse parameter may be selected from a group consisting of power, frequency, pulse duration, pulse shape, pulse duty cycle and time between pulses. The electrical characteristic may be related to reflective power. The controller may vary the power of each of the pulses of microwave energy. Controller may also be responsive to a control input from an operator for modifying any one of the at least one pulse parameters.

A method for applying microwave energy to target tissues includes the steps of positioning the microwave energy delivery device into or adjacent a portion of the target tissue and delivering at least two pulses of microwave energy to the target tissue wherein a substantial portion of the microwave energy is reduced between the at least two pulses of microwave energy. The delivering step may include the steps of heating a portion of the target tissue to a target temperature and maintaining the portion of the target tissue at or above the target temperature.

In another embodiment the step of delivering the at least two pulses of microwave energy may include the step of selecting and varying at least one parameter thereof. The step of selecting and varying the at least one parameter may further include the step of selecting the at least one parameter from a group consisting of power, frequency, pulse duration, pulse shape, phase, pulse duty cycle and time between pulses.

In yet another embodiment the step of delivering the at least two pulses of microwave energy may include the step of varying at least one of the at least two pulses of microwave energy in accordance with at least one characteristic of an electrical transient of one of the at least two pulses of microwave energy.

In yet another embodiment the step of delivering the at least two pulses of microwave energy may include the step of selecting parameters of the at least two pulses of microwave energy such that a rise in target tissue temperature is about zero.

In yet another embodiment the step of delivering the at least two pulses of microwave energy may include the steps of measuring at least one characteristic of the target tissue in response to one of the at least two pulses of microwave energy and in accordance with the at least one characteristic of the target tissue, determining whether to change the microwave energy parameters. The step of measuring the at least one characteristic of tissue may further include the step of selecting the at least one characteristic of the tissue from a group consisting of a characteristic related to reflective power, a characteristic related to tissue temperature, and a characteristic related to tissue impedance. The method may further include the step of determining a response of the target tissue to one of the at least two pulses of microwave energy.

In yet another embodiment the step of delivering the at least two pulses of microwave energy may include the steps of measuring at least one characteristic of one of the at least two pulses of microwave energy and in accordance with the at least one measured characteristic, determining whether to change at least one microwave energy parameter. The step of measuring the at least one characteristic of one of the at least two pulses of microwave energy may include the step of selecting at least one characteristic related to reflective power. The method may further include the steps of measuring at least one characteristic of one of the at least two pulses of microwave energy and in accordance with the at least one measured characteristic, at least one of determining whether to terminate the application of microwave energy to tissue, and using the at least one measured characteristic to determine a set of microwave energy parameters for a subsequent pulse of microwave energy. The at least one microwave energy parameter may be selected from a group consisting of power, frequency, pulse duration, pulse shape, phase, pulse duty cycle and time between pulses.

In yet another embodiment the method of positioning the microwave energy delivery device into or adjacent a portion of the target tissue and delivering at least two pulses of microwave energy may include the steps of measuring at least one characteristic of the target tissue in response to one of the at least two pulses of microwave energy; and in accordance with the at least one characteristic of the target tissue, at least one of determining whether to terminate the application of microwave energy, and using the at least one characteristic of the target tissue to determine a set of microwave energy parameters for applying a subsequent pulse of microwave energy. The set of microwave energy parameters for the application of a subsequent pulse of microwave energy may includes a magnitude of a microwave power, a pulse shape, pulse duration and any combination thereof.

The delivery of microwave energy may be terminated upon a determination that a predetermined condition is satisfied. The predetermined condition is selected from a group consisting of treatment duration, a condition related to temperature and a condition related to reflective power.

In yet another embodiment of the present disclosure a method for applying microwave energy to a target tissue, includes the steps of positioning a microwave energy delivery device into or adjacent a portion of the target tissue, delivering at least two pulses of microwave energy to the target tissue wherein a substantial portion of the microwave energy is reduced between the at least two pulses of microwave energy, measuring at least one of the temperature of a transmission line and a temperature of the microwave energy delivery device, and varying at least one of the at least two pulses of microwave energy in response to the at least one of the measured temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a system for supplying microwave energy for microwave therapy, according to an embodiment of the present disclosure;

FIG. 2 illustrates various waveforms associated with electrical characteristics of microwave therapy performed with the system for supplying microwave energy of FIG. 1;

FIG. 3 illustrates microwave pulses generated by the system for supplying microwave energy of FIG. 1 according to a method of the present disclosure; and

FIG. 4 illustrates microwave pulses generated by the system for supplying microwave energy of FIG. 1 according to another method of the present disclosure.

DETAILED DESCRIPTION

Embodiments of the presently disclosed microwave antenna assembly will now be described in detail with reference to the drawing figures wherein like reference numerals identify similar or identical elements. As used herein and as is traditional, the term “distal” refers to the portion of the device or instrument that is furthest from the user and the term “proximal” refers to the portion of the device or instrument that is closest to the user. In addition, terms such as “above”, “below”, “forward”, “rearward”, etc. refer to the orientation of the figures or the direction of components and are simply used for convenience of description.

Two factors in the treatment of diseased tissue with microwave therapy include the proper positioning and placement of a microwave antenna relative to the target tissue, and the delivery of microwave energy.

The step of positioning and placement of a microwave antenna, relative to target tissue, is dependant upon the location of the target tissue. One method of accessing target tissue involves the insertion of the microwave antenna into an existing body lumen, such as, for example, accessing the inner chambers of the heart through the vascular system, accessing the prostate by inserting a microwave antenna through the urethra or accessing the lungs and esophagus by inserting a microwave antenna through the mouth cavity. Various methods and devices are provided for in U.S. Pat. No. 5,916,241, entitled “Device and Method for Asymmetrical Thermal Therapy with Helical Dipole Microwave Antenna”, to Rudie et al., describing accessing the prostate through the urethra, and U.S. Pat. No. 7,070,595, entitled “Radio-Frequency Based Catheter System and Method for Ablating Biological Tissue”, to Ormsby et al., describing accessing cardiac tissue through the vascular system.

Another method of accessing target tissue involves direct insertion of a device into tissue. A microwave antenna may be formed or inserted through a delivery device such as, for example, an introducer or catheter, as described in U.S. Pat. No. 6,355,033, entitled “Track Ablation Devices and Methods”, to Moorman et al. The microwave antenna may also be sufficiently strong to allow direct insertion into tissue as described in U.S. Pat. No. 6,878,147, entitled “High Strength Microwave Antenna Assemblies”, to Prakash et al. Yet another microwave device may be inserted into tissue in a first condition followed by deployment of the distal penetrating portion thereof to a second condition thereby forming a microwave antenna at the distal end of the assembly as described in U.S. application Ser. No. 10/272,314, entitled “Microwave Antenna Having a Curved Configuration”, to Prakash et al.

The delivery of microwave energy is also dependant upon the location of the target tissue. Ormsby et al. '595 describes delivery of RF energy in the vascular system that generates and delivers a continuous train of radio frequency energy pulses at an output frequency for transmission in a transmission line to a shapeable RF antenna. The output frequency is adjusted based on the sensed reflective signal in order to substantially match the transmission line impedance with the shapeable RF antenna and the biological tissue load impedance. Rudie et al. '241 describes delivery of a continuous train of microwave energy in the urological system. In both Ormsby et al. '595 and Rudie et al. '241 the output frequency is adjusted to reduce the risk of overheating by the electrical transmission and to prevent damage to the lumen in which the transmission line is contained.

The present disclosure provides a system for supplying microwave energy and various methods of delivering microwave radiation to tissue with a microwave device. A microwave device of the system for supplying microwave energy may be inserted directly into tissue, inserted through a lumen, such as, for example, a vein, needle or catheter, placed into the body during surgery by a clinician or positioned in the body by other suitable methods or means known in the art. While the method described hereinbelow is targeted toward microwave ablation and the complete destruction of target tissue, the methods may be used with other microwave therapies in which the target tissue is partially damaged, e.g. in order to prevent the electrical conduction in heart tissue.

Referring now to FIG. 1, a system for supplying microwave energy for microwave therapy, according to an embodiment of the present disclosure, is shown as 10. The system for supplying microwave energy 10 includes an electrosurgical generator 20, with a controller 22 for controlling the operation of the electrosurgical generator 20, an electrosurgical instrument or device 30 coupled to an output 24 on the electrosurgical generator 20. Device 30 includes a microwave antenna 32, for delivery of microwave energy, coupled to a transmission line 34, which electrically connects antenna 32 to output 24 on electrosurgical generator 20. Electrosurgical generator 20 includes an operator interface 40 having a keypad 42 and a display 44 for entering microwave energy parameters by an operator.

Device 30 is illustrated as a microwave antenna sufficiently strong to allow direct insertion or penetration into tissue, although microwave antenna may be any device suitably capable of delivering microwave energy or the like.

System 10 may include one or more sensors to measure various parameters associated with the delivery of microwave energy. For example, device 30 and/or transmission line 34 may include one or more suitable temperature measuring sensor 35, 36. As seen in FIG. 1, a first sensor 35 is connected to transmission line 34 and a second sensor 36 is connected to antenna 32. Sensors 35, 36 may be incorporated into the construction of, or formed as part of, the transmission line 34 and/or antenna 32 and may connect to electrosurgical generator 20 through transmission line 34 and connector 24. Alternatively, sensors 35, 36 may be placed on, in or near transmission line 34 or antenna 32, such as, for example, a percutaneously insertable temperature or impedance sensor or a surface mountable temperature sensor and may connect directly to electrosurgical generator 20.

FIG. 2 illustrates various waveforms associated with electrical characteristics of microwave therapy performed with the system 10 of FIG. 1. A first waveform is forward power “FP” or power supplied to the antenna 32 by the electrosurgical generator 20. A second waveform is tissue temperature “T” measured by a temperature measuring sensor 36. A third waveform is reflected power “RP” or power that is not transmitted by the antenna 32 to tissue. Each waveform is related to the delivery of microwave energy.

During operation of system 10, as seen in FIG. 2, during an initial 40 seconds of operation, the temperature “T” of tissue begins to rise as energy is delivered to the tissue. Between 40 and 50 seconds of operation, the tissue temperature “T” exceeds 100° C. and the water or liquid in the patient tissue “PT” (see FIG. 1) begins to boil. Between 40 and 90 seconds of operation, tissue temperature “T” is maintained at a temperature slightly over 100° C. Between 90 and 100 seconds of operation, the water or liquid in the patient tissue “PT” is boiled off and the tissue temperature “T” again begins to rise. The energy supplied to patient tissue “PT”, from system 10, begins to char the tissue “PT” and potentially irreversible changes or effects result in the tissue “PT”.

FIG. 2 is provided as an example of waveforms associated with electrical characteristics of microwave therapy and should not be construed as limiting hereto. The systems and methods described herein may utilize other suitable waveforms associated with microwave therapy. The shape of the waveforms are dependent upon many factors (e.g., the type of tissue receiving the energy, the desired therapeutic result, the size and type of microwave antenna used and/or the power and frequency of the microwave energy.) For example, tissue with high fluid content, such as, for example, kidney tissue, liver tissue or cardiovascular tissue, may require more energy and time to heat than tissue with low fluid content, such as, for example, lung tissue.

The desired therapeutic result may also impact the shape of the individual waveforms. For example, during tissue ablation tissue temperature may rise to about 60° C. to 70° C. and the clinician may maintain the tissue at a temperature for a target amount of time. Other treatments may require raising tissue to a temperature less than 60° C. or may not require maintaining the tissue at a target temperature.

With continued reference to FIG. 2, electrosurgical generator 20 maintains forward power “FP” at approximately 60 Watts for the duration of the delivery of microwave energy.

During the initial delivery of microwave energy to patient tissue “PT”, between 0 and 70 seconds, the antenna 32 is matched to the tissue properties and reflective power “RP” is at a minimum at a level slightly more than zero. Between 90 and 100 seconds the properties of patient tissue “PT” begin to change and reflective power “RP” begins to increase. This increase in reflective power “RP” indicates that the antenna 32 is less “tuned”, or matched to the properties of tissue “PT”, and less power is delivered to tissue “PT”. At this point, electrosurgical generator 20 continues to maintain constant forward power “FP” and the antenna 32 become less efficient and reflective power continues to increase.

Reflective power may be reduced by adjusting the frequency of the delivered microwave energy to match the impedance of the damaged tissue. The present disclosure provides for a system and method of controlling the delivery of microwave energy in order to prevent and/or minimize a change in tissue impedance, thus preventing further increases in reflective power.

With reference to FIGS. 1 and 2, forward power “FP”, temperature “T” and reflective power “RP” are waveforms, or electrical transients, related to microwave energy delivery. Controller 22 may be responsive to at least one characteristic of an electrical transient, or characteristic that occurs during at least one individual pulse of microwave energy. The characteristic of an electrical transient, or property of a given waveform, may include the rate of change of reflective power “ΔRP”, rate of change of temperature “ΔT”, rate of change of impedance (not shown), rate of change of forward power “ΔFP” or a value from the waveform. The transient may also be related to a particular phenomenon that occurs in the waveform, such as, for example, the heating time “HT” of tissue in response to the delivery of microwave power or the reaction time of tissue temperature in response to the removal of the delivery and/or transmission of microwave energy.

Controller 22 may also be responsive to an individual measurement related to microwave energy delivery, such as, for example, an instantaneous or time weighted measurement of temperature, power or impedance.

With reference to FIGS. 1 and 3, controller 22 is operable to cause the electrosurgical generator 20 to apply two or more pulses 100-100 n of microwave energy to tissue. Each pulse of microwave energy is defined by various parameters such as, for example, frequency of the microwave energy, amount of power delivered by the microwave energy, the time duration of the microwave energy pulse, and the shape of the microwave energy pulse. For example, the pulse may be a square wave pulse 100, as illustrated in FIG. 1, a pulse with a magnitude or a pulse having a shape that may be dependant of the amount of energy delivered to tissue.

Controller 22 is operable to remove, reduce or turn off the delivery of energy 110-110 n to patient tissue “PT” between energy pulses 100-100 n. With reference to FIG. 1, removal of energy delivery to patient tissue “PT” may be accomplished by switching the delivery of energy from the output 24 to a secondary load 26. Energy may be reduced by automatically adjusting, or by substantially reducing, the power delivered to the output 24. Alternatively, controller may turn off the energy delivered to the output 24.

The energy pulse generated by the controller, irrespective of the method of generation, is characterized by delivery of energy to tissue for an amount of time followed by a second amount of time wherein the energy delivered to tissue is substantially reduced. Preferably, the energy delivered to tissue during the second amount of time is reduced to a level that is metabolically insignificant, or equal to the basal metabolism level of about 1 W/kg of tissue.

The controller 22 may include means for switching (not explicitly shown) the delivery of energy to secondary load 26. The means for switching may be a separate device included in the electrosurgical generator 20 or it may be an external device controlled by the electrosurgical generator 20. Secondary load may be a resistive load, for absorbing and/or dissipating the energy, an additional electrosurgical device, such as, for example, a microwave antenna, or any combination thereof.

With continued reference to FIG. 1, controller 22 measures an electrical characteristic of the microwave energy delivered to patient tissue “PT” and is capable of adjusting one or more pulse parameters based on the measured electrical characteristic. Measurements may be made or performed by the electrosurgical generator 20, the controller 22, a sensor 36 in the device 30, a sensor 35 in transmission cable 34 or by a separate sensor (not explicitly shown) coupled to the electrosurgical generator 20. Controller 22 may vary the pulse parameter of the current microwave energy pulse or may vary the pulse parameter of any subsequent energy pulse. Controller 22 may also utilize the measured electrical characteristics of microwave energy to determine if the energy delivery should continue or if energy delivery should be terminated.

FIG. 3 illustrates at least one embodiment of microwave energy delivered to patient tissue “PT” by the system 10 of the present disclosure. The microwave energy is defined by a series of energy pulses 100 to 100 n separated by delays 110-110 n during which delays 110-110 n the energy delivered to tissue “PT” is removed or is substantially reduced. First microwave pulse 100 is followed by a first delay 110, followed by one or more subsequent energy pulses 100 a-n each of which is separated by a delay 110 a-110 n−1.

Providing microwave energy as a series of pulses, separated by delays, wherein the energy is removed or substantially reduced, improves delivery and dispersion of heat within the target tissue. Removing power provides a relaxation period for the tissue “PT” and enables the tissue “PT” to re-hydrate and recover. Microwave energy is then reapplied to tissue “PT” after the delay or relaxation period and the process may be repeated as needed and/or desired. This periodic redistribution of heat through the tissue “PT” during the relaxation period, followed by the re-delivery and/or re-transmission of microwave energy, improves the ablation size and results in a more predictable ablation area.

Measurements may be continuously or periodically performed during energy delivery, the relaxation period or any combination thereof.

During the delivery of microwave energy, antenna mismatch or reflected power may produce standing waves in the transmission cable 34 and/or device 30 resulting in an increase in the voltage standing wave ratio (hereinafter “VSWR”). Standing waves produce hot spots in the transmission cable 34 at various locations along a length thereof, the hot spots are typically spaced every ½ wavelength along the cable. The location and/or the temperature of the hot spots correspond to a specific characteristic of the standing wave or to the value of the VSWR. Continuous delivery of microwave energy may result in these hot spots exceeding a maximum expectable temperature and may result in patient or clinician injury or damage of equipment.

Providing a series of microwave energy pulses separated by delays, as disclosed herein, may result in even distribution of heat along the transmission path. For example, the delay between consecutive pulses may allow the thermal energy to conduct away from each hot spot resulting in a more even distribution of temperature. In addition, pulsing of microwave energy may change at least one characteristic of the standing wave. For example, the standing wave may change phase or relaxation of patient tissue may change the VSWR. This may result in repositioning or shifting the hot spots between a pulse of microwave energy and a subsequent pulse. Shifting of the location of the hot spots between pulses may result in a more even distribution of heating and/or temperature along the transmission cable 34.

Microwave energy therapy parameters are used to determine the number of microwave energy pulses, the duration and power of each individual energy pulse, the delay between energy pulses and the total duration of the procedure. Individual pulse parameters include the parameters associated with each individual pulse, such as, for example, the duration of the pulse, the frequency of the pulse, the power setting of the pulse and the delay after each individual pulse. With reference to FIG. 1, parameters may be predetermined by a clinician and entered through the operator interface 40, may be determined by the controller 22 or may be entered through the keypad 42 by the clinician and adjusted by controller 22. Pulse parameters may be adjusted for each individual pulse. Alternatively, some parameters, such as the frequency of the microwave energy, may be consistent throughout the duration of the microwave energy therapy.

Controller 22 may be responsive to the measured electrical characteristics of one or more microwave energy pulses. The measured electrical characteristic may be related to reflective power, such as, for example, reflective power magnitude, reflective power phase, reflection loss, reflection coefficient, voltage standing wave ratio (VSWR), return loss, or mismatch loss VSWR. The measured electrical characteristic may be a sensed parameter such as, for example, temperature of the transmission line 34 measured by sensor 35 or the temperature of the device 30 measured by device sensor 36. In operation, controller 22 may adjust one or more parameters of the current energy pulse or of a subsequent microwave energy pulse.

In the operation of one embodiment of the present disclosure, a clinician may view the various electrical characteristics related to reflective power, via the user interface 40 and display 44. The clinician may make adjustments to the microwave energy therapy parameters or pulse parameters based on the various electrical characteristics. The clinician may adjust one or more parameters of the current pulse or of a subsequent pulse or may adjust one or more microwave energy therapy parameters.

A first method of applying microwave therapy to tissue includes the steps of applying a first pulse of microwave energy to tissue, removing a substantial portion of the microwave energy from the tissue and applying at least one subsequent microwave energy pulse to tissue. Removing a substantial portion of the microwave energy may include: reducing the energy to a level that allows the tissue relaxes, reducing the energy to a level that maintains tissue temperature, reducing energy to a level that is metabolically insignificant, or reducing the delivered energy to zero.

As discussed hereinabove and with reference to FIG. 1, controller 22 may be responsive to at least one characteristic of an electrical transient that occurs during at least one individual pulse of microwave energy or may be responsive to a measurement related to microwave energy delivery. The step of applying at least one subsequent microwave energy pulse to tissue may include the step of selecting and varying at least one parameter for the subsequent microwave energy pulse. The parameter may be varied in accordance to an individual measurement or may be varied in accordance with at least one characteristic of an electrical transient that occurs during an individual pulse of microwave energy. The varied parameter may include, and is not limited to forward power, frequency, energy, pulse duration, pulse shape and termination of energy application.

FIG. 4 provides an example of energy delivery in accordance with one embodiment of the present disclosure. With reference to FIGS. 1 and 4, initial energy pulse 200 is applied with a forward power setting of 80 watts. For the initial energy pulse 200, controller 22 may be responsive to a characteristic of tissue “PT”, a characteristic of the microwave energy pulse and/or a characteristic of an electrical transient. Controller 22 terminates the initial energy pulse 200 when a rate of change of tissue temperature “T” exceeds a threshold value. The threshold value of the rate of change of tissue temperature may be a predetermined value, an operator entered value and/or a value calculated by controller 22. Alternatively, duration of the initial energy pulse 200 may be shortened or a pulse parameter, such as power, may be modified. In FIG. 4, the rate of change of temperature “T” terminates the initial energy pulse 200 at time “T1”.

After the termination of the initial energy pulse 200, the temperature “T” may continue to rise for a period to time “T2” due to the heat dispersing within the target tissue. The duration of a first off period 210 having a period of time “T3” that may be equal to a predetermined amount of time. Controller 22 may vary the duration of the first off period 210 in response to a characteristic of tissue, a characteristic of a previous microwave energy pulse and/or a characteristic of an electrical transient. In FIG. 4, the first off period 210 terminates at time “T4” and the first subsequent energy pulse 200 a is applied to the target tissue.

The individual pulse parameters of the first subsequent energy pulse 200 a may be varied according to a characteristic of tissue, a characteristic of the microwave energy pulse or a characteristic of an electrical transient, such as, for example, the duration of the first off period 210. A long “off” period may indicate that the target tissue contains excessive heat energy and the individual pulse parameter may be adjusted to deliver less energy. Alternatively, a short “off” period may indicate that the target tissue quickly disperses the heat energy in the target tissue and the energy of the first subsequent energy pulse should remain the same or may be increased.

Control of the first subsequent energy pulse 200 a may be similar to the control of the initial energy pulse 200 or control may be responsive to a different measured parameter or different characteristic of an electrical transient. As the temperature “T” of tissue “PT” increases, steam begins to form at the molecular level and/or the tissue begins to transform. Steam generation and phase transformation effect thermal delivery and dispersion within the target volume and reflective power “RP” will begin to increase. At this point, parameters of the subsequent energy pulses 200 c-2001 are selected as to not appreciably heat the tissue “PT”.

With continued reference to FIG. 4, reflective power “RP” may remain relatively constant during the initial delivery of electrosurgical energy, e.g. between 0 and about 45 seconds. As the temperature of the tissue increase, e.g. between about 45 seconds and about 60 seconds, the reflective power “RP” increases. When the reflective power “RP” exceeds a threshold value, the energy delivery may be modified, for example, the power level may be decreased and/or the duration between energy subsequent energy pulses may be increased. The threshold value for reflective power “RP” may vary between about 1% and about 20% of reflective energy, typically about 5% of reflective energy.

A further embodiment of the present method includes the step of measuring at least one characteristic of the microwave energy pulse and determining whether to change the microwave energy parameters. The characteristic may be related to reflective power “RP”, such as, for example, the instantaneous measurement or the rate of change of reflective power “ΔRP”. Energy pulse parameters are selected as to control or maintain selected characteristics.

Reflective power “RP” and the rate of change of reflective power “ΔRP” can be used as an indicator of the condition of tissue. Formation of steam, while conducive to thermal delivery of heat, removes moisture from tissue and changes the impedance of the tissue. A change of tissue impedance creates an imbalance between the antenna and the tissue “PT” which causes an increase in reflective power “RP”. In selecting parameters to maintain or reduce reflective power “RP”, the system 10, while creating some steam, does not damage tissue “PT” resulting in more predictable and/or larger ablation sizes.

According to a further method of the present disclosure, at least one characteristic of the tissue “PT” is measured in response to the applied first pulse. The controller 22, in accordance with the measured characteristic may terminate the delivery of microwave energy to tissue. The controller 22 may also use the measured characteristic to determine a set of microwave energy parameters for a subsequent microwave energy pulse. The steps are then repeated for the subsequent pulses.

In a further embodiment of the present method, the set of microwave energy parameters for the subsequent microwave energy pulse may include at least one of a magnitude of a starting microwave power, pulse shape, pulse duration and any combination thereof.

In a further method of the present disclosure, the delivery of electrosurgical energy, and/or the “off” period, may be adjusted for temperature management of portions of the electrosurgical system or temperature management of patient tissue “PT”. With reference to FIG. 1, controller 22 of the electrosurgical generator 20 monitors at least one of the temperature of the transmission line 34 via line sensor 35 and/or the temperature of the device via device sensor 36. Device 20 may include one or more tissue temperature measuring sensors (not shown) or tissue measurement sensors may connect directly to electrosurgical generator. With reference to FIG. 4, energy delivery may be intermittently reduced or paused, or the “off” period may be increased, to allow the transmission line 34, device 20 or patient tissue “PT” to cool.

The system for supplying microwave energy for microwave therapy and the methods of use discussed above are not limited to microwave antennas used for hyperthermic, ablation, and coagulation treatments but may include any number of further microwave antenna applications. While several embodiments of the disclosure have been shown in the drawings and/or discussed herein, it is not intended that the disclosure be limited thereto, as it is intended that the disclosure be as broad in scope as the art will allow and that the specification be read likewise. Modification of the above-described system and methods for using the same, and variations of aspects of the disclosure that are obvious to those of skill in the art are intended to be within the scope of the claims. Therefore, the above description should not be construed as limiting, but merely as exemplifications of particular embodiments. 

What is claimed is:
 1. A method for applying microwave energy to a target tissue of a body, comprising the steps of: positioning a microwave energy delivery device into and adjacent a portion of target tissue; delivering at least two pulses of microwave energy to the target tissue; measuring a temperature of a portion of a transmission line external to the body, the transmission line configured to transmit the at least two pulses of microwave energy to the microwave energy delivery device; measuring a temperature of the microwave energy delivery device; and varying at least one parameter related to the step of delivering the at least two pulses of microwave energy to the target tissue in accordance with at least one of the measured transmission line temperature and the measured microwave energy delivery device temperature.
 2. The method as in claim 1, wherein one of the at least one parameter is an off time between two of the at least two pulses of microwave energy.
 3. A method as in claim 1, wherein the step of varying the at least one parameter includes the step of selecting the at least one parameter thereof.
 4. A method as in claim 3, wherein the step of selecting and varying the at least one parameter includes the step of selecting the at least one parameter from a group consisting of power, frequency, pulse duration, pulse duty cycle and time between pulses.
 5. A method as in claim 1, wherein one of the at least one parameter is related to a forward power waveform.
 6. A method as in claim 1, wherein the step of delivering the at least two pulses of microwave energy include the steps of: measuring at least one characteristic of the target tissue in response to one of the at least two pulses of microwave energy; and in accordance with the at least one characteristic of the target tissue, determining whether to change the microwave energy parameters.
 7. A method as in claim 1, further comprising the step of terminating the delivery of microwave energy upon a determination that a predetermined condition is satisfied.
 8. A method as in claim 7, wherein the predetermined condition is selected from a group consisting of treatment duration, a condition related to temperature and a condition related to reflective power. 